In general, a phosphor is a material that exhibits luminescence, which is generally the generation of light in excess of that which is radiated thermally. Luminescence is the result of the competition of radiative and non-radiative pathways in the relaxation of an electronically excited species. Luminescence encompasses many different excitation pathways, such as, but not limited to, excitation caused by electrons (cathodoluminescence), excitation caused by light (photoluminescence), excitation caused by X rays or particles (scintillation), and excitation by electrons and holes (electroluminescence). Furthermore, depending on the relaxation time, scale is broken down into scintillation, spontaneous emission, fluorescence, phosphorescence, etc.
Many phosphors belong to the lanthanide group, also termed the rare earth elements. If one succeeds in introducing these rare earth ions (also called dopants and/or activators) into a particular host material, be it a crystal, a glass, a liquid, a molecular material such as a polymer, the host material may become luminescent. Practically speaking, however, a luminescent material must be easily excited by the appropriate excitation, have a high quantum efficiency, i.e., the ratio of the number of quanta absorbed to the number emitted, convert the energy absorbed to a useful frequency of visible light (if the use is in the visible), and maintain its usefulness under both excitation mode and ambient usage conditions, and, ideally, is easily manufactured.
Commercial prior art phosphors are mostly inorganic compounds prepared as powders (with grain sizes usually in the order of 2-20 μm) or thin films. The phosphor materials contain one or more impurity ions or activators, typically present in 0.01-100 mol % concentrations, with the actual light emission generated by these activator ions.